Symbol generating tube having target matrix with conducting elements



Nov. 8, 1966 R. WINFIELD 3,234,658

SYMBOL GENERATING TUBE HAVING TARGET MATRIX WITH CONDUCTING ELEMENTS Filed Dec. 50, 1965 5 Sheets-Sheet l 'l'.\/v MONITOR HORIZONTAL SWEEP V INVENTOR. F I 4 Rama/v0 W/NF/ELD BY/g A TTOR/VE) Nov. 8, 1966 R. WINFIELD RATING TUBE HAVING TARGET SYMBOL GENE MATRIX WITH CONDUCTING ELEMENTS Filed Dec.

5 Sheets-Sheet 2 INVENTOR. RA YMOND W/NF/EL 0 MEHZFEOOO 0 mCKQZZEOOO ATTORNEY NOV- 8, R WINF|ELD I SYMBOL GENERATING TUBE HAVING TARGET MATRIX WITH CONDUCTING ELEMENTS Filed Dec. 30, 1963 a SheetsSheet :5

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IN VEN TOR.

RA YMOND l V/NF/EL 0 ATTORNEY United States Patent 3,284,658 SYMBOL GENERATING TUBE HAVING TARGET MATRIX WITH CONDUCTING ELEMENTS Raymond Winfield, Wantagh, N.Y., assignor to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed Dec. 30, 1963, Ser. No. 334,393 11 Claims. (Cl. 315-21) This invention relates to symbol display systems and to symbol generating cathode ray tubes for use in such systems.

Applications frequently arise in which letters, numerals, or other symbols must be superimposed on a cathode ray tube display.

In air tralfic control displays, for instance, radar information is often displayed in the form of a TV picture on a cathode ray tube. Aircraft identifying symbols are superimposed on the TV picture so that the progress of individual aircraft can be followed without confusion.

According to one prior art scheme for accomplishing this purpose, a TV camera pickup is focused on identifying symbols. The output signals from this camera are then mixed with video signals derived from the radar return for presentation on a common TV monitor. This system becomes unwieldy, however, when a large number of targets must be tracked. Furthermore, with this system .it is difficult to maintain the symbol at the desired position on the TV monitor with a high degree of accuracy.

Another common method used to display symbol information together with radar or TV data involves the ,use of a two-gun electrostatic cathode ray display tube. One gun is used exclusively for symbols and the other for radar or 'TV information. The electron beams from both guns excite a common display screen. However, there is a limit to the brightness and resolution of the TV pattern obtainable in this system. This in turn limits the quality of the display.

It is an object of the present invention to provide means for electronically superimposing a large number of symbols .on acathode ray tube monitor.

Another object of the present invention is to provide means for accurately positioning an electronically superimposed symbol on a cathode ray tube monitor.

Yet another object of the present invention is to provide means for superimposing symbols on a cathode ray tube monitor with a .high degree of brightness and resolution.

Still another object of the present invention is to provide means for generating symbols on a cathode ray tube monitor regardless of the scanning pattern used with the monitor.

These and other objects are achieved by providing a special cathode ray tube in which a target element, shaped to conform to the symbol to be generated and placed in the path of the electron beam, can be switched to a potential such that it will permit electrons to escape to a collector element when the target is traversed by the electron beam of the cathode ray tube. The electrical output from the collector element of this cathode ray tube can be used to intensity modulate a TV monitor and the sweep of the cathode ray tube can be synchronized with that of the TV monitor.

The principles and operation of the invention may be better understood by referring to the following description and the accompanying drawings.

FIG. 1 is a diagram of a symbol generating tube useful in practicing the invention,

FIG. 2 is a diagram of a target matrix that may be employed in the tube of FIG. 1,

FIG. 3 is a circuit diagram of a display system employing the tube of FIG. 1,

v Patented Nov. 8, 1966 FIG. 4 is .a diagram representing certain wave shapes used in the circuit of FIG. 3,

FIG. 5 is a diagram useful in explaining the operation of the invention,

FIGS. 6-8 are diagrams representing further embodiments of the tube of FIG. 1, and

FIG. 9 is a diagram of a target matrix that may be used in the embodiment of FIG. 8.

Referring now to FIG. 1, an elongated tube envelope 11 is fitted with .a conventional electron gun 13. Electrically conducting target elements 15 and 17 are also placed inside the tube envelope. These target elements are fabricated from a conducting material but insulated from each other. External leads are connected to each of these elements. A collector grid 19 is placed near these target elements. The collector grid is preferably made from a conducting mesh material similar to that commonly used for grid elements in conventional cathode ray storage tubes. An external connection is provided so that a voltage V may be applied to the collector and so that an output voltage may be taken from this grid. Conventional deflection means such as the deflection plates 21 permit the electron beam to be swept across the target elements in any desired scanning pattern.

In operation, the cathode of the tube would normally be grounded. A positive voltage typically in the range of 50 to 300 volts would be applied to the collector 19. Provisions for switching the individual target elements between a small negative potential and a small positive potential would be included in the external circuits associated with the tube. Assume that the upper target element 15 is switched to a first condition by being biased to a small negative potential and that the lower target element is switched to a second condition by being biased to a small positive potential. When the electron beam sweeps over the target element 15, the electrons will approach this target element, but will reverse direction and return to the positively charged collector element. These electrons will flow to the source of collector voltage. When the electron beam sweeps over the lower target element 17, however, the electrons will be attracted to this element since it is biased to a positive voltage with respect to the cathode. Since these electrons are not returned to the collector, the collector current will de crease. The difference in current between the two states constitutes the useful signal current. The electron beam can be made .to scan the target elements according to known scanning techniques. The electron beam will give rise to an output voltage whenever the beam traverses a portion of a properly biased target element. When the beam has completely scanned a target element, it will have produced a voltage wave uniquely related to the shape of the target element. By switching the individual target elements between the two bias states, various combinations of output signals can be obtained.

A target matrix may be conveniently patterned after one of the numerous types of numeric and alpha-numeric configurations known in the display art. One of these configurations is depicted in FIG. 2. As applied to the tube of FIG. 1, this configuration would be made from a number of conducting target elements insulated from each other and placed inside the tube as indicated in FIG. 1. Each conducting target element would contain an external terminal which could be connected to suitable biasing or switching circuits.

Since the difference in collector current produced by the two bias states constitutes the useful signal output, either bias state may arbitrarily be considered as producing the on condition of the tube and the opposite bias state will then be considered as producing the oif condition of the tube;

Assume thatthe low collector current condition has been chosen as the on condition.

If a letter T were to be generated, the upper horizontal target element 23 and the vertical element 25 would be biased on by means of a positive voltage. All other target elements would be biased off by means of a suitable negative voltage. On the first horizontal sweep of the electron beam, electrons would be attracted by the element 23 and diverted from the collector grid thus producing an on output signal during this interval. At subsequent sweeps, the electron beam would sweep horizontally across the target matrix at successively lower positions. An on output signal would appear only during the intervals when the beam was sweeping across the element 25. The output voltage would thus be a unique representation of the letter T. A variety of other symbols could be generated in the same fashion by biasing selected combinations of the target elements.

Various other configurations, devised for use in electrooptical character display systems, are shown in US. Patent 3,067,413 issued to Liselotte Fischle et al., on December 4, 1962.

Furthermore, known forms of dot matrices may be employed if so desired.

FIG. 3 illustrates a circuit that may be used with the tube of FIG. 1 to generate symbols for a TV monitor. This circuit employs a symbol generating tube 27 which provides an output signal that can be mixed with TV information in a video mixer 29. The output of the video mixer is used to intensity modulate a conventional TV monitor 31. A synchronization generator 33 is used to time the horizontal and vertical sweep circuits of the TV monitor. The output of the synchronization generator is also used to trigger the horizontal and vertical comparison sweep generators 35 and 37. The outputs of the sweep circuits 35 and 37 are applied to the sym ol channels 39, 41 and 43. These symbol channels are identical so that only the first symbol channel 39 has been shown in detal in FIG. 3. Any number of symbol channels (designated as channels 1 through N) may be used in the circuit.

The symbol channels each contain a horizontal voltage comparator 45 which is connected to the horizontal comparison sweep generator 35. Similarly, each channel contains a vertical voltage comparator 47 which is connected to the output of the vertical comparison sweep generator 37. A second input of each of these comparator circuits is connected to a computer 49. The computer produces a pair of coordinate voltages for each symbol channel in the system.

Since any spot position on the TV monitor may be defined by a vertical and a horizontal deflection voltage, a pair of coordinate voltages may be produced by the computer to correspond to any spot position on the TV monitor. Assuming that the sweep circuits of the TV moni tor move the spot from left to right and from top to bottom, a particular pair of coordinate voltages from the computer may be used to define the upper left hand corner of the region in whicha symbol is to be displayed on the TV screen.

The output of the horizontal and vertical comparators is applied through a comparator output AND gate 51 to a character height circuit 53. Thus when the voltages from the vertical and horizontal comparison sweep generators 35 and 37 reach the value of the corresponding coordinate voltages produced by the computer 49, a pulse will be applied to the character height circuit 53. The character height circuit produces an elongated pulse shown as curve A in FIG. 4. This circuit may comprise a conventional pulse stretcher or one-shot multivibrator which produces an output pulse having a predetermined duration as will be explained.

A pulse from the horizontal comparator 45 is also ap plied to a symbol channel AND gate 55 each time that the output of the horizontal comparison sweep circuit matches the X coordinate voltage from the computer 49. The AND gate 55 thus produces an output pulse every time the horizontal comparison sweep voltage reaches a predetermined value during the interval that a pulse is being produced by the character height circuit 53. This can be visualized by referring to curves A and B of FIG. 4, wherein curve A represents the output of circuit 53 and curve B represents the series of trigger pulses from gate 55. The output pulses from the AND gate 55 are applied to a symbol channel OR gate 57, and to a vertical position memory circuit 59. Similar vertical position memory circuits 61 and 63 are connected to receive output pulses from symbol channel 2 and symbol channel N respectively.

The pulse output from the AND gate 55 is applied through the target channel OR gate 57 to a horizontal sweep generator 73. The pulses applied to this generator initiate horizontal sweep voltages for the symbol generating tube 27. These sweep voltages are pictured in curve C of FIG. 4. The sweep voltages C have a duration which is a predetermined portion of the corresponding TV monitor horizontal sweep voltage. Thus the duration of the horizontal sweep applied to the symbol generating tube determines the width of the symbol to be displayed on the monitor.

A horizontal line of the target matrix must be scanned each time that the trace on the TV monitor passes through the region in which a symbol is to be generated. Since the elongated output pulse from the character height circuit 53 determines which comparator pulses will be available to trigger the horizontal sweep generator 73, the character height circuit must produce an elongated pulse having a duration equal to the time necessary for the trace on the TV monitor to pass over a vertical height equal to the desired symbol height. For instance, if a symbol is to occupy a height equal to the height of 16 adjacent scanning lines on the TV monitor, the duration of the character height output pulse must be sufi'lcient to permit sixteen trigger pulses to be applied to the horizontal sweep generator 73.

The vertical position memory circuits contain either a ramp or a staircase generator as preferred. A pulse applied to the memory circuit from the AND gate 5 triggers the generator so that it provides an output wave beginning concurrently with, and having a duration equal to, that of the character height generator 53. This output wave is pictured as curve D of FIG. 4. The outputs of the vertical position memory circuits are applied to the memory gates 65, 67 and 69 respectively. These gates are also connected to receive enabling pulses from the input terminals of the corresponding position memory. They combine the functions of an AND gate and a pulse stretcher so as to provide an output pulse having a duration equal to the horizontal sweep time used for the symbol generating tube, and having an amplitude equal to the instantaneous output voltage of the associated vertical position memory circuit whenever a trigger voltage is applied to that memory circuit. Typically, these memory gates may contain a one shot multivibrator and an AND gate. The multivibrator is switched by a trigger pulse so as .to enable the AND gate for the specified time.

The outputs from the various vertical memory circuits are applied through a vertical memory OR gate 71 to the vertical deflection plates of the symbol generating tube.

The vertical sweep generating circuits can be arranged to produce any scanning sequence such as an interlaced scan according to well-known techniques. In general, the scanning sequence used for the symbol generating circuit should be compatible with the scanning sequence for the TV monitor.

A character matrix element selector 75 is connected to apply bias voltages to the target elements 77 and 79 so that these elements may attract or repel electrons so as to produce the desired signals. The selector 75 contains a logic circuit that determines which of the target elements will be energized in response to commands from the computer. Command signals are received through a bank of input terminals designated as 1, 2, and N from the corresponding input terminals of the re spective vertical memory circuits. A collector 81 senses current changes as the beam sweeps over the target elements thus producing an output signal which is amplified and conducted to the video mixer 29.

' To better understand the operation of this circuit, consider a case in which a numeral 7 and a numeral 9 are to be displayed on a monitor as depicted in FIG. 5.

Assume that the numeral 9 is positioned below the numeral 7 to such an extent that four horizontal sweeps of the TV monitor will occur before the trace enters the region in which the numeral 9 is to be displayed. Assurne further that target channel 1 has been arranged to read out a numeral 7 and target channel 2 has been arranged to read out a numeral 9.

The computer 49 will be adjusted to produce a first pair of coordinate voltages X Y and a second pair of coordinate voltages X Y These coordinate voltages will have a magnitude equal to the magnitude of the output voltages of the horizontal and vertical comparison sweep circuits 35 and 37 when the spot on the TV monitor reaches the upper left hand corner of the respective regions to be occupied by the desired symbols.

When the horizontal and vertical comparators 45 and 47 each detect a comparison with the X Y coordinates, the character height generator 53 will be triggered. A pulse will be applied to the OR gate 57 and to the first vertical memory circuit 59. The output from the OR gate 57 will initiate a horizontal sweep in the generator 73 causing the electron beam of the symbol generating tube to begin a horizontal sweep. At the same time, the first pulse applied to the vertical memory circuit 59 will trigger the generator in this circuit and cause a rectangular pulse to be applied to the vertical deflection plates of the symbol generating tube 27. This will establish the vertical position of the electron beam in the tube 27 during its first sweep of the numeral 7.

The pulse applied to the vertical memory circuit is also applied to the channel 1 input terminal of the character matrix element selector 75. This circuit in turn energizes appropriate target elements so that an output signal from the symbol generating tube corresponding to the first horizontal sweep of the desired numeral will be applied to the video mixer 29.

When the electron beam of the symbol generating tube has completed its first horizontal excursion, it will return to its quiescent position. During this time, however, the electron beam in the TV monitor continues on its first horizontal excursion, and the wave generator in the vertical memory circuit 59 continues to produce a triangular vertical deflection voltage as depicted in curve D of FIG. 4.

When the beam in the TV monitor reaches the end of the first sweep, it returns to the left of the screen and begins a second horizontal sweep. When the deflection of this beam again reaches the value X another pulse is produced by the horizontal comparator 45. Since the character height generator 53 is still producing an elongated pulse (curve A), the pulse from the horizontal comparator passes through the gates 55 and 57 to initiate a second horizontal sweep in the signal generating tube 27 and to the memory gate 65 to permit another pulse (the second pulse of curve E) to reach the vertical defiection plates of the tube 27.

Since the output voltage of the vertical position memory has been steadily increasing during the entire readout process, the rectangular pulse applied to the vertical deflection plates now has a magnitude such that the electron beam of the symbol generating tube sweeps over a second horizontal path on the target matrix. An output signal is produced by the symbol generating tube corresponding to the bias of the target elements in this path.

The process continues during the third and fourth horizontal sweeps of the TV monitor beam in the region of the numeral 7. On the fifth sweep, however, this beam will traverse the screen at a level that also contains a portion of the region in which the numeral 9 is to be dis played. As the TV monitor beam proceeds on this particular sweep, it first causes a readout of a fifth portion of the numeral 7. As the TV monitor beam leaves the region containing the numeral 7 the electron beam in the symbol generating tube returns to its quiescent position.

When the TV monitor spot reaches the point at which the numeral 9 is to be displayed, the comparator circuits in target channel 2 will detect a comparison with the X Voltage from the computer and pass an output signal to the OR gate 57, the vertical memory circuit 61, and the channel 2 input terminal on the character matrix element selector 75. Target elements in the symbol generating tube 27 corresponding to the numeral 9 will be energized.

The pulse applied to the vertical memory circuit 61 will trigger the wave generator in that circuit and produce a first output pulse of the type shown in curve B of FIG. 4. The electron beam of the symbol generating tube will sweep across the target in a horizontal plane at a level determined by this memory circuit output voltage so as to produce a first scan along the top of the target elements selected for producing the numeral 9.

When the electron beam of the symbol generating tube has completed this scan, it will again return to its quiescent position. However, the spot on the TV monitor will continue to the end of this fifth horizontal sweep and begin its sixth horizontal sweep. When this spot again reaches the horizontal deflection at which the numeral 7 is to be displayed, the horizontal sweep of the symbol generating tube will be triggered. The vertical memory circuit 59 will still be generating the original triangular wave which now has a value such that it causes the electron beam in the symbol generating tube to sweep horizontally across the target matrix at the sixth level of the numeral 7. Thereafter, the target elements corresponding to the numeral 9 are scanned at their second level.

This process is repeated until the desired number of horizontal sweeps have scanned the entire target matrix for each symbol to be generated.

By the time that the beam of the TV monitor has passed below the region in which the numeral 7 is generated, the elongated pulse (curve A) from the character height generator in symbol channel 1 will have terminated. This prevents the formation of subsequent trigger pulses from target channel 1 as the TV beam passes the X deflection. The character height generator in the symbol channel 2 continues to produce an elongated pulse, however, so that the information pertaining to the lower portion of the numeral 9 can still be read out of the symbol generating tube.

Although the circuit description has concerned an arrangement in which each symbol channel provides a single digit, additional logic circuits may be provided so that a target channel will produce a given multiple digit number.

Although sweep generators using triangular wave shapes have been described, it is to be understood that known types of digital sweep circuits may be used if so desired.

Since the entire diameter of the target is used for each sweep of a generated symbol, and since each symbol is a small fraction of the TV monitor diameter, drift effects or other beam positioning errors arising in the symbol generating tube cause negligible errors in the TV monitor display.

Many variations of the basic symbol generating tubes of FIG. 1 will be apparent to those skilled in the art.

Typical of these variations is the symbol generating tube shown in FIG. 6. This tube contains an elongated envelope 111. A conventional electron gun 113 is positioned near one end of the envelope and the target elements 115 and 117 are mounted inside the envelope opposite the electron gun. A collector 119 similar to the collector used in the tube of FIG. 1 is mounted between the electron gun and the target elements. This collector is maintained at a positive voltage typically in the order of 400 volts. Deflection electrodes 121 are mounted near the electron gun so as to control the positioning of the electron beam. An additional decelerator grid 123 is interposed between the selector and the target elements. This decelerator grid is normally maintained at a positive volt-age in the range of 50 to 300 volts and is bypassed to ground.

The decelerator grid acts to decelerate the electron beam rapidly and uniformly in the region between the grid and the target.

The decelerator grid reduces the capacitance between the target elements and the collector. This shields the collector so that any switching transients arising when the target elements are switched from one bias state to the other do not influence the collector output signal.

Another embodiment of the basic symbol generating tube is pictured in FIG. 7. In this embodiment, a tube envelope 211 contains a conventional electron gun 213 and target elements 215 and 217 similar to those previously described. The collector 219 in this embodiment, however, consists of a concentric conducting cylinder. This cylinder may be formed as a separate structure or from a conducting coating placed on the inside of the tube envelope. This tube also contains a decelerator grid 223. Operation of the tube of FIG. 7 is similar to the operation of the tube of FIG. 6. The embodiment of FIG. 7, however, can be designed to still further reduce the switching transients since the capacitance between the target elements and the collector can be minimized in this embodiment.

A presently preferred embodiment of the symbol generating tube is pictured in FIG. 8. In this embodiment, a tube envelope 311 contains an electron gun 313. The target 315 in this tube consists of an insulating disc containing a series of apertures 317. The tube also contains a decelerator grid 319, conventional deflection plates 321, and an accelerator grid 323. The collector element 325 is placed beyond the various grids and the target matrix.

The structure of the target may be better visualized by referring to FIG. 9. An insulating disc 327 contains metallic target elements 329 disposed in any desired pattern. These target elements are insulated from each other and contain suitably spaced apertures 317.

A typical target may be formed from a thin ceramic disc. The matrix elements may be formed by applying a metallic coating in the desired areas of the ceramic disc. The insulating gaps between individual target elements may be a small fraction of the width of a target element. The apertures may be spaced and dimensioned so as to occupy about 50% of the area of the target element.

During normal operation of the tube, the cathode conveniently may be grounded. The decelerating grid may be maintained at a positive voltage in the range of 50-300 volts and the accelerating grid may be maintained at a positive voltage in the range of 10-100 volts. The target elements may be switched between an off value of -2 volts so as to turn back electrons and an on value of about +10 volts so as to allow electrons to penetrate the apertures and ultimately reach the collector electrode.

In this embodiment of the tube, the signal collector may be spaced farther from the target matrix than is possible in any of the other embodiments. The switching transients may be reduced accordingly. The decelerator grid i319 reduces the velocity of the electrons reaching the target elements. The accelerator grid 323 serves to pull electrons through. the apertures when the target elements are properly biased. The accelerator grid also acts to decouple the target matrix from the collector element so as to further reduce any switching transients that might occur.

Because of the low beam energy in the vicinity of the target elements in each of the embodiments of the tube, high speed, low voltage switching circuits may be used to switch rapidly from one target element combination to another.

Although the various embodiments of the tube ordinarily contain a single target matrix, application mayarise in which it would be desirable to have two or more matrices arranged side by side in a given tube. For instance, it may be desirable to have a tube in which various geometric symbols might be arranged adjacent to an alpha-numeric target matrix. The same principles govern the construction of these multiple target tubes.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are Words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A symbol generating electron tube comprising an elongated tube envelope; an electron gun positioned near one end of the tube envelope and arranged to provide an electron beam along the axis of the tube envelope; a target matrix arranged transversely in the path of said electron beam; means to sweep the electron beam across the target matrix; electrically conducting target elements in said matrix; means to supply on and off control voltages to the individual target elements whereby varying numbers of electrons are allowed to escape from the target matrix in response to the applied control voltage; and signal output means to collect electrons escaping from the target.

2. A symbol generating electron tube comprising a tube envelope; an electron gun to provide an axial beam inside said envelope; a target matrix disposed transversely across the electron beam; individual conductive target elements in said matrix, said elements being shaped to conform to portions of the symbols to be generated; deflection means to sweep the electron beam across the target matrix; means to collect electrons escaping from the target matrix as the beam sweeps over the matrix; means to produce an output signal in response to the number of electronscollected; and means to alter the potential of individual target elements so as to control the number of electrons escaping from any target element.

3. A symbol generating electron tube comprising an elongated cathode ray tube; an electron gun disposed coaxially in one end of the tube; a substantially flat matrix target disposed transversely within the tube; a flat metallic mesh within said tube disposed parallel to said matrix target and intermediate the target and the electron gun; an exterior collector terminal connected to said metallic mesh; individually insulated metallic target elements in said matrix target, said target elements being shaped to correspond to portions of the symbol to be generated; and an individual exterior target element terminal connected to each target element.

4. A symbol generating electron tube comprising an elongated tube envelope; means positioned near one end of said envelope to form an electron beam flowing toward the opposite end of the envelope; conductive target means positioned near the opposite end of the tube envelope to capture the electrons in said beam; means to alter the potential of the target means so that varying proportions of the total number of electrons in the beam will be captured by the target; means to collect the electrons not captured by the target means; output means to provide a voltage proportional to the electrons collected by said collecting means; and means to sweep the electron beam transversely across the target means.

5. A symbol generator comprising an evacuated electron tube envelope; an electron gun to provide a beam of electrons within the tube envelope; a cathode in said electron gun; means to establish a cathode potential; a target matrix electrode positioned transversely in the electron beam; a collector electrode positioned intermediate the target electrode and the electron gun; deflection means to sweep the electron beam over the target electrode; an exterior terminal on said collector electrode; a source of collector potential that is positive with respect to the cathode potential; an impedance means connecting said terminal to said collector source of potential; individually insulated matrix elements in said target electrode, each of said elements being shaped to conform to a portion of a symbol to be generated; a first target potential source having a magnitude intermediate the cathode potential and the collector potential; a second target potential source having a magnitude substantially equal to the cathode potential; and target matrix selection means to switch individual target matrix elements between said first and second target potential sources whereby the beam electrons sweeping past a target matrix element will be captured .by this element when it is connected to said first target potential source and repelled back toward the collector electrode when this element is connected to said second target potential source.

6. The symbol generator of claim 5, wherein the collector electrode is a fiat metallic mesh disposed'parallel to the matrix target electrode.

7. The symbol generator of claim 5, wherein the collector electrode is a conductive ring disposed annularly within the tube envelope.

8. The symbol generator of claim 7, wherein the conductive ring is coated on the inside of the tube envelope.

9. A symbol generating electron tube comprising an elongated tube envelope; an electron gun positioned near one end of the tube envelope and arranged to provide an electron .beam along the axis of the tube envelope; a target matrix arranged transversely in the path of said electron beam; a conductive target matrix element in said target matrix, said target matrix element containing a series of perforations having their axes substantially parallel to the axis of the elongated tube envelope; means to sweep the electron beam across the target matrix; means to collect those electrons from the electron beam that pass through said perforations; and means to apply electrical bias to said matrix element so as to control the number of electrons passing through said perforations.

10. A symbol generating electron tube comprising an elongated tube envelope; an electron gun positioned near one end of the tube envelope and arranged to provide an electron beam along the axis of the tube envelope; at substantially fiat target matrix arranged transversely in the path of said electron beam; conductive matrix elements in said target matrix, each of said matrix elements being shaped to correspond to a portion of a symbol to be generated, each of said matrix elements further containing a series of perforations having their axes substantially aligned with the electron beam; means to sweep the electron beam across the target matrix; means to sup ply on and off bias voltages to individual matrix elements whereby selectively controlled numbers of electrons are allowed to pass through said perforations as the electron beam sweeps over each matrix element; and a collector element positioned beyond the target matrix and electrically charged to a potential suitable for attracting those electrons that pass through the perforations.

11. A symbol generating electron tube comprising an elongated tube envelope; an electron gun positioned near one end of the tube and arranged to provide an electron beam substantially along the axis of said tube envelope; a target matrix disposed transversely in the electron beam; individual conductive matrix elements in said target matrix, said matrix elements each being shaped to correspond to the shape of a portion of a symbol to be generated, each of said matrix elements further containing a series of apertures; a decelerator grid interposed between the electron gun and the target matrix and maintained at a positive potential with respect to the cathode; deflection means to sweep the electron beam across the target matrix; means to alter the potential of individual target elements so that varying numbers of electrons from the electron beam can pass through the apertures; a collector placed beyond the target matrix and maintained at a positive potential with respect to the cathode so that the electrons passing through said apertures are attracted to the collector; and an accelerating grid interposed between the target matrix and the collector and maintained at a voltage inter-mediate that of the collector and the most positive bias voltage applied to the matrix elements.

DAVID G. REDINBAUGH, Primary Examiner. T. A. GALLAGHER, Assistant Examiner. 

1. A SYMBOL GENERATING ELECTRON TUBE COMPRISING AN ELONGATED TUBE ENVELOPE; AN ELECTRON GUN POSITIONED NEAR ONE END OF THE TUBE ENVELOPE AND ARRANGED TO PROVIDE AN ELECTRON BEAM ALONG THE AXIS OF THE TUBE ENVELOPE; A TARGET MATRIX ARRANGED TRANSVERSELY IN THE PATH OF SAID ELECTRON BEAM; MEANS TO SWEEP THE ELECTRON BEAM ACROSS THE TARGET MATRIX; ELECTRICALLY CONDUCTING TARGET ELEMENTS IN SAID MATRIX; MEANS TO SUPPLY ON AND OFF CONTROL VOLTAGES TO THE INDIVIDUAL TARGET ELEMENTS WHEREBY VARYING NUMBERS OF ELECTRONS ARE ALLOWED TO ESCAPE FROM THE TARGET MATRIX IN RESPONSE TO THE APPLIED CONTROL VOLTAGE; AND SIGNAL OUTPUT MEANS TO COLLECT ELECTRONS ESCAPING FROM THE TARGET. 